Keel joint arrangements for floating platforms

ABSTRACT

Keel joint assemblies are described that permit a degree of rotational movement of a riser within the keel of a floating vessel and greatly reduce the amount of stress and strain that is placed upon the riser, as well. Keel joint assemblies described provide a limiting joint between the riser and the keel opening that permits some angular rotation of the riser with respect to the floating vessel. Additionally, the limiting joint permits the riser to move upwardly and downwardly within the keel opening, but centralizes the riser with respect to the keel opening so that the riser cannot move horizontally with respect to the keel opening. In described embodiments, the limiting joint is provided by a single annular joint that allows that riser to move angularly with respect to the can. In some embodiments, the keel joint assembly incorporates a cylindrical stiffening can that radially surrounds a portion of the riser and is disposed within the keel opening. In these embodiments, a flexible joint is provided between the can and the riser. Supports or guides may be used to retain the can within the keel opening.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the priority of provisional patentapplication serial No. 60/308,365 filed Jul. 27, 2001.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates generally to methods and devices forproviding a stress-relieving joint between a riser and the keel of afloating platform.

[0004] 2. Description of the Related Art

[0005] Deep water floating platforms use risers to communicateproduction fluid from the sea floor to the floating production platform.Floating platforms have a portion that lies below the surface of thesea. For stability of the platform, it is desired that there be a verydeep draft. The spar, for example, is a popular style of floatingplatform that has an elongated, cylindrical hull portion which, whendeployed, extends downwardly a significant distance into the sea. Thelowest portion of the submerged hull is referred to as the keel.Currents in the sea tend to move the floating platform laterally acrossthe sea surface. Despite the presence of anchorages, the platformimparts bending stresses to the riser during lateral movement.Localized, or point, stresses are particularly problematic for risers.

[0006] One known joint arrangement for use with risers and floatingvessels is described in U.S. Pat. No. 5,683,205 issued to Halkyard.Halkyard describes an arrangement wherein a joint means is positionedwithin a keel opening in the floating vessel to reduce the amount ofstress upon a pipe passing through the keel opening. The joint meansconsists of a radially enlarged sleeve member with an elastomericannulus at either end that is in contact with both the sleeve member andthe pipe. Halkyard's intent is to reduce stress upon the pipe that isimposed by lateral movement of the floating vessel upon the sea. Inorder to reduce stress, Halkyard contacts the pipe at two points with anelastomeric annulus, which is described as providing a resilient,somewhat yieldable connection. Unfortunately, Halkyard's arrangement isproblematic since it permits almost no angular movement of the pipewithin the sleeve member. While point stresses upon the pipe arereduced, they are still significant. Further, the pipe is required tobend within the confines of the sleeve. This bending, together with theinduced point stresses at either end of the sleeve, place significantstrain on the pipe.

[0007] The present invention addresses the problems in the prior art.

SUMMARY OF THE INVENTION

[0008] Keel joint assemblies are described that permit a degree ofrotational movement of a riser within the keel of a floating vessel. Theassemblies of the present invention greatly reduce the amount of stressand strain that is placed upon the riser, as well. The present inventiondescribes keel joint assemblies that provide a limiting joint betweenthe riser and the keel opening that permits some angular rotation of theriser with respect to the floating vessel. Additionally, the limitingjoint permits the riser to move upwardly and downwardly within the keelopening, but centralizes the riser with respect to the keel opening sothat the riser cannot move horizontally with respect to the keelopening.

[0009] In described embodiments, the limiting joint is provided by asingle annular joint that allows that riser to move angularly withrespect to the can. In some embodiments, the keel joint assemblyincorporates a cylindrical stiffening can that radially surrounds aportion of the riser and is disposed within the keel opening. In theseembodiments, a flexible joint is provided between the can and the riser.Supports or guides may be used to retain the can within the keelopening.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 illustrates an exemplary riser extending upwardly from thesea floor and through a spar-type floating platform.

[0011]FIG. 2 is a schematic side, cross-sectional view of a firstexemplary keel joint assembly constructed in accordance with the presentinvention.

[0012]FIG. 3 is a schematic side, cross-sectional view of a secondexemplary keel joint assembly constructed in accordance with the presentinvention.

[0013]FIG. 4 is a schematic side, cross-sectional view of a thirdexemplary keel joint assembly constructed in accordance with the presentinvention.

[0014]FIG. 5 is a schematic side, cross-sectional view of a fourthexemplary keel joint constructed in accordance with the presentinvention.

[0015]FIG. 6 is a schematic side, cross-sectional view of a fifthexemplary keel joint assembly constructed in accordance with the presentinvention.

[0016]FIG. 7 is a schematic side, cross-sectional view of a sixthexemplary keel joint assembly constructed in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017]FIG. 1 generally illustrates a subsea wellhead 10 that has beeninstalled into the sea floor 12. A riser 14 is connected to the wellhead10 and extends upwardly through the waterline 16 to a floating platform18. The riser 14 is used to transmit production fluids or as a drillingconduit from the wellhead 10 to production facilities (not shown) on thefloating platform 18. The riser 14 is used to provide a closed conduitfrom the wellhead 10 to the floating platform 18. The floating platform18 shown is a spar-type floating vessel that carries productionequipment (not shown) on an upper deck 20. The hull 22 of the platform18 is a cylinder having flotation chambers within and a central,vertically-oriented passage 24 through which the riser 14 is disposed.It is noted that the configuration for a passage used in floatingplatforms varies from platform to platform. Sometimes the passage islined by a cylindrical wall that extends substantially the entire lengthof the hull. In other platforms, the passage is partially lined by sucha wall, and in still other platforms, there is essentially no lining forthe passage. The keel 26 is located at the lower end of the hull 22. Akeel joint, indicated generally at 28, is used to permit axial upwardand downward motion as well as angular deflection of the riser 14 withrespect to the keel 26. It is desired that the keel joint 28 beconstructed to preclude localized bending stresses in the riser 14 thatcould damage it, resulting in structural failure of the riser 14.

[0018] Referring to FIG. 2, there is shown a first, and currently mostpreferred, exemplary keel joint arrangement 30 that can be used as thekeel joint 28 to support the riser 14. The keel joint arrangement 30includes a stiff cylindrical can 32 that radially surrounds a portion ofthe riser 14. The can 32 is retained within and disposed away from thewalls of the keel opening or passage 24 by supports or guides 34 thatare securely affixed with the hull 22. While there are only two upperand two lower supports 34 shown in FIG. 2, it should be understood thatthere are actually more such supports 34, perhaps four or more upper andfour or more lower supports 34 and that the supports are located tosurround the circumference of the riser 14. The supports 34 haverounded, non-puncturing ends 36 to contact the outer wall of the can 32.It is noted that the supports 34 are not affixed to the can 32, therebypermitting the can 32 to move upwardly and downwardly within the passage24. The keel joint arrangement 30 maybe thought of an “open can”arrangement since the can 32 is affixed to the riser 14 by a stressjoint (straight or tapered) 38 proximate the lower end of the can 32while the upper end 40 of the can 32 is not secured to or maintained incontact with the riser 14. The exemplary stress joint 38 illustratedconsists of a pair of radially enlarged collars 42 that surround theriser 14 and are affixed to the inner radial surface of the can 32. Thecollars 42 are shown to be fashioned of metal. However, the collars 42may also be fashioned of a suitable elastomeric material. The collars 42may be substantially rigid so as to permit a small amount of angularmovement of the riser 14 with respect to the can 32. Alternatively, thecollars 42 may be relatively flexible to permit additional angularmovement.

[0019] In operation, the riser 14 can move angularly to a degree withinthe can 32 under bending stresses. Illustrative directions of suchrelative angular movement are shown in FIG. 2 by arrows 33 aboutrotation point 35. During such angular movement, the outer walls of theriser 14 are moved closer to or further away from the inner walls of thekeel opening 24. The stress joint 38 forms a fulcrum. The can 32 isstiff enough that it transfers stresses directly from the stress joint38 to the supports 34, thereby preventing any significant stresses frombeing seen by the upper portion of the riser 14. Generally, thisarrangement allows the upper portion of the riser 14 to have a smallercross section than the stress joint 38.

[0020]FIG. 3 illustrates an alternative embodiment for a keel jointarrangement 50 that is useful as a keel joint 28. In the keel jointarrangement 50, a heavy walled wear sleeve 52 radially surrounds aportion of the riser 14. The wear sleeve 52 may or may not be secured tothe riser 14 in a fixed relation, such as by the use of welding orretaining rings such as are known in the art. A central portion of thewear sleeve 52 has an external annular ring 54 that extends radiallyoutwardly and forms the portion of the sleeve 52 having the largestexterior diameter. The ring 54 presents an outer radial surface that isvertically curved in a convex manner. The outer radial surface of thering 54 may also be frustoconical in shape. Below the annular ring 54 isa lower inwardly tapered portion 56. Above the ring 54 is an upperinwardly tapered portion 58. A partially-lined passage, designated as24′, in the hull 22 of the floating vessel 18 has an open upper end 60that is outwardly flared for installation purposes. The flare of theupper end assists in guiding the sleeve 52 and ring 54 into place whenlowering the riser 14 through the hull 22. The lower end of the passage24 has an annular recess 62 that is sized and shaped for the annularring 54 to reside within. The recess 62 presents an inner surface thatis vertically curved in a concave manner so that the outer convexsurface of the annular ring 54 can be matingly engaged. If the outerradial surface of the ring 54 is frustoconical in shape, however, theinner surface of the recess 62 will be made complimentary to thatfrustoconical shape.

[0021] In operation, the keel joint arrangement 50 helps to preventdamage to the riser 14 from bending stresses. The wear sleeve 52 islocated at the keel 26 where the primary bending stresses are impartedto the riser 14 and, therefore, is designed to absorb most of thosestresses and prevent them from being imparted directly to the riser 14.The interface of the ring 54 and the recess 62 provides a fulcrumwherein the riser 14 can move angularly with respect to the hull 22. Inaddition, the elongated upper tapered portion 58 will tend to bearagainst the length of the passage 24′, thereby reducing or eliminatinglocalized, or point, stresses.

[0022] Referring now to FIG. 4, there is shown a keel joint arrangement70, which is a second alternative embodiment that is useful as the keeljoint 28. The keel joint arrangement 70 employs centralizer assemblies72 that are secured within the passage 24 of the hull 22. Preferably,the centralizer assemblies 72 are spaced angularly about thecircumference of the passage 24. In a preferred embodiment, thecentralizers 72 comprise hydraulically actuated piston-type assemblies,the piston arrangement being illustrated schematically by two 72 a, 72b. In practice, the two arms 72 a, 72 b would be nested one within theother in a piston fashion and would be selectively moveably with respectto one another. In an alternative embodiment, the centralizer assemblies72 comprise hinged assemblies wherein the two arms 72 a, 72 b arehingedly affixed to one another at hinge point 72 c. Actuation of thecentralizer assembly in this case would move the arm 72 a angularly withrespect to the arm 72 b about the hinge point 72 c, thereby permittingthe arm 72 a to be selectively moved into and out of engagement with theriser 14. The centralizers 72 are energized via hydraulic lines (notshown) to urge the riser toward the radial center of the passage 24 toresist contact between the riser 14 and the passage 24. The centralizers72 have rounded, non-puncturing tips 74 that bear upon the riser 14.Preferably, the non-puncturing tips comprise either wear pads or rollersfor engagement of the riser 14. It is noted that the piston-typecentralizer assemblies 72 may be actuated mechanically rather thanhydraulically. Also, the centralizer assemblies' attachments to thepassage 24 may be softened, such as through use of springs or rubber, insuch a way as to decrease bending stresses by yielding to riserdeflection. In a further alternative embodiment, the centralizers 72will comprise members that have a hinged attachment to the passage 24.

[0023]FIG. 5 depicts a third alternative embodiment for the keel joint28. Keel joint assembly 90 includes a riser collar 92 that surrounds aportion of the riser 14 proximate the keel 26. The collar 92 is notaffixed to the riser 14 but instead permits sliding movement of theriser 14 upwardly and downwardly through the collar 92. The collar 92 isgenerally cylindrical but includes a bulbous central portion 94 and twotapered end portions 96, 98. A guide sleeve 100 radially surrounds thecollar 92 and features an interior rounded profile 102 that is shapedand sized to receive the bulbous portion 94 of the collar 92. Anexterior landing profile 104 is located at the lower end of the guidesleeve and is shaped and sized to form a complementary fit with alanding profile 106 formed into the keel 26. The passage 24′ isconstructed identically to the passage 24′ described earlier in that ithas an open upper end with an outward flare.

[0024] To assemble the keel joint arrangement 90, the collar 92 andguide sleeve 100 are assembled onto the riser 14. Then the riser 14 isrun through the passage 24′ and the landing profile 104 of the guidesleeve 100 is seated into the matching profile 106 in the keel 26. Inoperation, the riser 14 can slide upwardly and downwardly within thecollar 92 as necessary to compensate for movement of the floatingplatform 18. Rotation of the platform 18 with respect to the riser 14 ispermitted between the riser 14 and the collar 92 as well as between thecollar 92 and the guide sleeve 100. Angular movement of the riser 14with respect to the platform 18 is accommodated by rotation of thebulbous portion 94 within the rounded profile 102 of the guide sleeve100. Alternatively, a rubberized flex joint of a type known in the art(not shown) might be used to accommodate angular rotation.

[0025] A fourth alternative exemplary embodiment for the keel joint 28is shown in FIG. 6. Keel joint assembly 110 incorporates a flexible cageassembly to permit relative movement between the riser 14 and thefloating vessel 18. A flexible cage assembly 112 is formed of an innerriser sleeve 114 and an outer keel sleeve 116. A central cage 118adjoins the two sleeves 114, 116. The cage 118 includes an upper ring120, a central ring 122, and a lower ring 124. There are a series ofupper spokes 126 that radiate upwardly and outwardly from the centralring 122 to the upper ring 124. There are also a series of lower spokes128 that radiate outwardly and downwardly from the central ring 122 tothe lower ring 124. The upper and lower spokes 126, 128 are eacharranged in a spaced relation from one another about the circumferenceof the central ring 122. The spokes 126, 128 are fashioned from amaterial that is somewhat flexible yet has good strength under bothtension and compression. It is currently preferred that the spokes 126,128 are fashioned of a steel alloy, although other suitable materialsmay be used. The spokes 126, 128 are elastically deformable as necessaryto allow the riser 14 to move angularly within the passage 24′. Angulardeflection of the riser 14 results in non-uniform deflection of upperspokes 126 and lower spokes 128. The upper ring 120 affixes the upperspokes 126 to the outer keel sleeve 116. The lower ring 124 is notaffixed to the outer keel sleeve 116.

[0026] The outer keel sleeve 116 is seated within the passage 24′ bymeans of a landing profile 130 that is shaped and sized to be seatedwithin a complimentary seating profile 132 at the lower end of thepassage 24′. Locking flanges 134 are secured onto the lower side of thekeel 26 to secure the outer keel sleeve 116 in place. In a manner knownin the art, the locking flanges 134 may be selectively disengaged, orunlocked, and subsequently retrieved by upward movement of the riser 14with respect to the passage 24′, i.e., by pulling upwardly on the riserstring.

[0027] During operation, the cage 118 holds the riser 14 in a semi-rigidmanner that permits some flexibility. The riser 14 can move angularlywith respect to the hull 22 due to the flexibility of the spokes 126 and128 of the cage 118. Loading from movement of the riser 14 istransferred by the upper spokes 126 to the keel sleeve 116 which, inturn transfers the loading to the hull 22. Because the keel sleeve 116engages the passage 24′ of the hull 22 along substantially its entirelength, point loading is avoided.

[0028]FIG. 7 depicts a fifth alternative embodiment for use as the keeljoint 28. Keel joint arrangement 130 includes an open top can structure,which is shown incorporated into the riser 14 as a sub 132 at is affixedat either end to other riser sections 134, 136. The can sub 132 includesa pair of concentric tubular members. The inner tubular member 138 hasthe same interior and exterior diameters as a standard riser section.The outer tubular member, or can, 140 is coaxial with the inner tubularmember 138 and is affixed to the inner tubular member 138 by a flangeadapter, or stress joint, 142 that joins the two pieces togetherproximate the lower end of the sub 132. While FIG. 7 shows the flangeadapter 142 to be an annular metallic collar that is integrally formedinto both the inner and outer tubular members 138, 140, it might alsocomprise a separate collar or elastomeric member as well as a flexiblecasing.

[0029] A cylindrical guide sleeve 144 radially surrounds the open topcan sub 132. The guide sleeve 144 is securely affixed to the outertubular member 140 by, for example, welding. Supports 146 are used tosecure the guide sleeve 144 within the passage 24 of the hull 22. Thesupports 146 maintain the guide sleeve 144 a distance away from the wallof the passage 24 so that the guide sleeve 144 is substantially radiallycentered within the passage 24. The supports 146 are preferably formedof structural beams. The supports 146 are arranged in two tiers, anupper tier and a lower tier, and each tier surrounds the circumferenceof the passage 24. The outer tubular member 140 is stiff enough that ittransfers stresses directly from the flange adapter 142 to the guidesleeve 144. Because the guide sleeve 144 and the outer tubular member140 are affixed along substantially their entire length, point stressesare avoided. In addition, the supports transmit loads or stresses fromthe guide sleeve 144 to the passage 24 walls. The length of contactbetween the outer tubular member 140 and the guide sleeve 144 allows fora longer vertical riser stroke than arrangements wherein there is lesscontact area, such as the arrangement 30 shown in FIG. 2.

[0030] While described in terms of preferred embodiments, those of skillin the art will understand that many modifications and changes may bemade while remaining within the scope of the invention.

What is claimed is:
 1. A keel joint for retaining a riser within thekeel of a floating vessel, the keel joint comprising: a keel openingdisposed generally vertically within a floating vessel; a riser disposedwithin the keel opening; and a limiting joint securing the riser withrespect the keel opening, the limiting joint precluding horizontalmovement or the riser with respect to the keel opening, but permittingvertical and angular movement of the riser with respect to the keelopening.
 2. The keel joint of claim 1 wherein the limiting jointcomprises: a generally cylindrical stiffening can that is disposedwithin the keel opening; and a collar that annularly surrounds the riserand is secured to the stiffening can.
 3. The keel joint of claim 1wherein the limiting joint comprises: a wear sleeve radially surroundingan outer portion of the riser; an annular ring disposed upon the outerportion of the wear sleeve, the ring presenting a vertically curvedouter surface; an annular recess formed within the keel opening andpresenting an inner surface that is generally complimentary to thecurved outer surface of the annular ring, the annular recess retainingthe annular ring therewithin to permit angular movement of the wearsleeve within the keel opening.
 4. The keel joint of claim 3 wherein thewear sleeve and collar are run in with the riser.
 5. The keel joint ofclaim 3 wherein the wear sleeve is seated within the keel opening usinga seating profile.
 6. The keel joint of claim 1 wherein the limitingjoint comprises: a wear sleeve radially surrounding an outer portion ofthe riser; an annular ring disposed upon the outer portion of the wearsleeve, the ring presenting a frustoconical outer surface; an annularrecess formed within the keel opening and presenting an inner surfacethat is generally complimentary to the frustoconical outer surface ofthe annular ring, the annular recess retaining the annular ringtherewithin to permit angular movement of the wear sleeve within thekeel opening.
 7. The keel joint of claim 6 wherein the wear sleevecomprises a tapered portion that assists angular movement of the wearsleeve within the keel opening.
 8. The keel joint of claim 6 wherein thekeel opening further comprises an outwardly flared upper portion.
 9. Thekeel joint of claim 1 wherein the limiting joint comprises at least onecentralizer assembly disposed within the keel opening and in contactwith the riser.
 10. The keel joint of claim 9 wherein the centralizerassembly comprises a piston-type centralizer assembly.
 11. The keeljoint of claim 9 wherein the centralizer assembly comprises a hingedcentralizer assembly.
 12. The keel joint of claim 9 wherein thecentralizer assembly is hydraulically-actuated.
 13. The keel joint ofclaim 9 wherein the centralizer assembly is mechanically-actuated. 14.The keel joint of claim 1 wherein the limiting joint comprises: a risercollar that radially surrounds a portion of the riser, the riser collarhaving a bulbous central portion; and a guide sleeve secured within thekeel opening to radially surround the riser collar, the guide sleevecomprising an interior rounded profile that is shaped and sized toreceive the bulbous portion and permit angular rotation of the bulbousportion therewithin.
 15. The keel joint of claim 14 wherein the guidesleeve and riser collar are run in with the riser.
 16. The keel joint ofclaim 14 wherein the guide sleeve is seated within the keel openingusing a seating profile.
 17. The keel joint of claim 14 furthercomprising a locking flange that secures the position of the guidesleeve with respect to the keel opening.
 18. The keel joint of claim 17wherein the locking flange may be unlocked and retrieved by upwardmovement of the riser with respect to the guide sleeve.
 19. The keeljoint of claim 1 wherein the limiting joint comprises: an inner,cylindrical riser sleeve that radially surrounds and engages the riser;an outer, cylindrical keel sleeve that radially surrounds the risersleeve and engages the keel opening; and a cage formed of a plurality offlexible spokes that radiate from the riser sleeve to the keel sleeve,the spokes being elastically deformable as needed to accommodate angularmovement of the riser with respect to the keel opening.
 20. The keeljoint of claim 19 further comprising a locking flange that secures theposition of the guide sleeve with respect to the keel opening.
 21. Thekeel joint of claim 20 wherein the locking flange may be unlocked andretrieved by upward movement of the riser with respect to the guidesleeve.
 22. The keel joint of claim 1 wherein the limiting jointcomprises: an open top can riser section that is incorporated into saidriser, the open top can riser section comprising: an elongated,cylindrical inner tubular member having end connection means forconnecting the inner tubular member to adjoining riser sections; anouter tubular member that radially surrounds the inner tubular memberand is secured within the keel opening; and an annular flange adapterthat interconnects the inner and outer tubular members.
 23. A keel jointfor retaining a riser within the keel of a floating vessel, the keeljoint comprising: a riser; a substantially cylindrical stiffening candisposed within a keel of a floating vessel and radially surrounding theriser; a single joint in contact with the can and the riser, the jointpermitting angular movement of the riser with respect to the can; and aplurality of supports extending from a keel of a floating vesselradially inwardly to contact the can, the supports being unaffixed tothe can to permit upward and downward movement of the can within thekeel of the floating vessel.
 24. The keel joint of claim 23 wherein thejoint comprises a pair of collars interconnect the can and riser. 25.The keel joint of claim 23 wherein the supports each present a roundedend for contacting the can.
 26. The keel joint of claim 23 wherein thesupports each present a roller for contacting the can.
 27. A keel jointfor retaining a riser within the keel of a floating vessel, the keeljoint comprising: a keel opening disposed generally vertically within afloating vessel; a riser disposed within the keel opening; and alimiting joint securing the riser with respect to the keel opening, thelimiting joint precluding horizontal movement or the riser with respectto the keel opening, but permitting vertical and angular movement of theriser with respect to the keel opening, the limiting joint comprising: awear sleeve secured to the riser; a stiffening can disposed radiallywithin the keel opening; and an annular member joining the wear sleeveto the stiffening can.
 28. The keel joint of claim 27 wherein theannular member comprises a cage having a plurality of radially extendingspokes.
 29. The keel joint of claim 27 wherein the annular membercomprises a collar.
 30. The keel joint of claim 27 further comprising aplurality of supports extending radially inwardly from the keel openingto contact the stiffening can.